This invention relates to threading apparatus, and the like, and more particularly to an improved thread roll attachment therefor, and a novel method of rolling threads onto bar stock and the like.
Heretofore a variety of methods have been employed to form threads on the end of cylindrical or tubular bar stock, or the like. One of the most basic ways, of course, is to utilize a lathe of the type in which a sharp tool point is fed perpendicularly to the work axis in order to cut a thread into the work. An alternative way of forming threads on the periphery of a workpiece is to utilize thread rolling apparatus of the type disclosed, for example in U.S. Pat. Nos. 2,345,871 and 4,766,750. These two patents teach the use of threading rolls, which have formed on their outer peripheral surfaces annular, generally V-shaped ribs that are urged under considerable pressure into rolling engagement of the outer peripheral surface of a piece of rotating bar stock. The ribs thus form threads on the bar stock by plastically deforming and displacing portions of the metal engaged by the thread rolling ribs.
Because of the excess amount of pressure which must be applied to the peripheral surface of a workpiece in order to "roll" a thread into the surface of a workpiece, it has been customary heretofore to use a plurality of threading rolls for effecting thread formation. For example, as disclosed in the above-noted U.S. Pat. No. 2,345,871, threads are formed on the outer peripheral surface of a tubular workpiece by expanding one end of the bore in the workpiece radially outwardly and into engagement with three threading rolls. The rolls are rotatably mounted in a surrounding head mechanism for rotation about parallel axes which are equiangularly spaced 180.degree. about the axis of rotation of the workpiece. Thus, during thread formation the periphery of the workpiece is engaged simultaneously at three, equiangularly spaced points about its axis by the three threading rolls. In the case of the above U.S. Pat. No. 4,766,750, threads are rolled onto the outer peripheral surface of a solid, cylindrical piece of bar stock by two threading rolls, which simultaneously are introduced radially into engagement with diametrally opposite sides of a rotating workpiece so that neither one roll nor the other tends to create a moment of force transversely of the axis of the rotating workpiece.
Accordingly, because of the extreme radial pressure which usually is applied to a rotating workpiece during the thread rolling of threads thereon, it has been customary in the past to utilize a plurality of thread rolls which engage the workpiece at equiangularly points about its axis of rotation. In that way, the axial centerline of the workpiece is stabilized during the thread rolling operation --i.e. it is not subject to any excess moment forces extending transverse to its axis. However, the use of plural rollers adds to the overall expense of the equipment, not only because of the cost of the rollers, but also because of the need for developing equipment which will accurately and simultaneously engage and disengage the threading rolls with the workpiece during a thread forming operation.
The present invention, however, provides a more simplified and less expensive method of rolling threads onto a workpiece by using a single thread roller to perform the same function heretofore performed by a plurality of such thread rolls. Although U.S. Pat. No. 3,877,273 teaches that single forming rolls have been utilized in the past for the cold rolling of gear teeth in a disc-shaped gear blank, there was no need or concern for balancing the radial pressure applied to the periphery of the gear blank, since the blank was of an axial length no greater than the axial length of the gear rolling tool. Also, as shown by U.S. Pat. No. 4,706,483, a single finishing or burnishing roll has been utilized to burnish a helical thread previously formed on a workpiece, but the radial pressure exerted by the burnishing wheel is minimal compared to that necessary for a thread rolling operation. In addition, as disclosed in U.S. Pat. No. 3,972,212, it heretofore has been customary to employ single knurling tool or roll for applying a knurled surface to the outer periphery of a rotating workpiece, but again, the forming pressure applied by the knurling roll to the work is insignificant as compared to the pressures heretofore normally applied by threading rolls to a workpiece.